1. Field of the Invention
This invention relates to a system and process for integrated desalting, deasphalting and desulfurization of hydrocarbon feedstocks.
2. Description of Related Art
In conventional oil refinery operations, various processes occur in discrete units and/or steps. This is generally due to the complexity of naturally occurring crude oil mixtures, and the fact that crude oil feedstocks processed at refineries often differ based on the location of the production well, the age of the production well, pre-processing activities at the production well, and the means used to transport the crude oil from the well to the refinery plant. Three very important and conventionally separate preliminary refining processes include desalting to reduce the level of inorganic salt dissolved in water entrained in the crude oil, desulfurization to reduce the organosulfur compounds present, and solvent deasphalting to separate the relatively heavy asphaltenic materials from a lighter deasphalted phase which is further refined into various petroleum products including transportation fuels. Prior methods or attempts to combine these processes are unknown to the inventors.
With respect to the salt content, a wide range of inorganic salt concentrations and compositions can be present in crude oil from sources in different parts of the world. The geologic formations from which crude is extracted influence the brine composition and concentration. Salt concentrations vary from merely brackish waters to highly concentrated solutions. The inorganic salt content of crude oils from the well can be in the range about 10-100,000 parts per million by weight (ppmw).
Salt in crude oil is dissolved in entrained water droplets. The salt composition in the brine can vary significantly. This is evidenced by wide ranges of calcium, sodium, magnesium, chloride, sulfate, and carbonate contents measured in crude oil brines around the world. For a specific crude oil mixture, salt content may correlate with bottoms, sediment, and water (“BS&W”) content. However, such relationships have little significance for different crude oil mixtures, or for crude oil mixtures from the same geologic formation that are recovered using different production techniques.
Production of crude from new oilfields generally commences with inorganic salt concentrations of only a few ppmw. As the age of the production well increases, however, the crude salt content rises. Water flooding and CO2 injection are principal secondary recovery techniques for continuing crude production from wells with declining crude flow. Crude oil produced by water flooding have higher than normal solids content and electrical conductivity, and are, therefore, more difficult to desalt. Injection of CO2 containing gases tends to dissolve more calcium bicarbonate into the water entrained in the crude oil. These secondary recovery techniques, therefore, while increasing the overall productivity, impact the crude oil salt concentrations.
When secondary recovery becomes uneconomical, tertiary recovery methods are used, including steam injection and fireflooding. Fireflooding involves injecting air in the producing well and igniting it to stimulate the flow of crude and increase recovery. Crude oil from tertiary recovery operations, particularly fireflooding, is notoriously difficult to desalt.
Fundamental problems associated with salt contaminants in crude oil feedstocks include heat exchanger fouling, plugging and corroding of refinery equipment such as piping and pipe stills. Furthermore, chemical processing of the feedstock can be hindered by sodium poisoning of catalysts. Salts also promote problematic furnace tube fouling and coking. Therefore, crude oil desalting processes have been developed to minimize or avoid these problems.
Initial dehydration of crude oil from a production well usually takes place in the oilfield to reduce the volume of water moving through the transport system. Most crude oils can be electrostatically dehydrated to the 0.1% to 0.5% BS&W range. Some heavier, e.g., having an American Petroleum Institute gravity value of less than 20°, and more viscous crude oils, e.g., possessing a viscosity greater than about 18 Centistokes at the operating temperature, i.e., the initial electrostatic dehydration vessel in the oilfield, can only be reduced to the 0.5% to 5.0% BS&W range. Depending on the source of the crude oil, the amount of salt that is typically acceptable for export markets is about 30 ppmw. While this is not low enough to control the fouling and corrosion which is ultimately required in a refinery, it is sufficiently low such that most single-stage desalting operations can achieve acceptably low salt concentration levels.
The primary function of a desalting process is to remove this salt from water droplets in the oil. Other contaminants, such as sediment, which can also promote heat exchanger fouling, plugging, erosion, and residual product contamination, are also removed in a desalter. In general, desalting operations wash the crude oil feedstock with fresh water, and subsequently remove the water to provide dry, low salt crude oil. Electrostatic desalters are commonly used to create an electrical field which acts on the water droplets to enhance coalescence. Electrostatic desalting is also used to remove other particulates from crude oil. The mixture of crude oil and brine is contacted with fresh water using a mix valve upstream of a desalter vessel. Salt is extracted from the brine into the wash water droplets. Demulsifiers are often added to enhance contacting effectiveness, droplet coalescence, and water separation. The electric field in electrostatic desalters enhances water droplet coalescence so that water/oil separation requires much less residence time, and hence a smaller vessel, as compared to settling operations without the imposed electric field.
Crude oil typically also includes significant amounts of asphaltenic and resinous materials, which are used as asphalt cement. Solvent deasphalting is a well known process to separate asphaltenes and resins after atmospheric and vacuum distillation. The pitch product contains the majority of the contaminants of the residue, including metals, asphaltenes, and Conradson carbon, and is rich in aromatic compounds.
In general, the feed is mixed with light paraffinic solvents having a chain of 3-7 carbon atoms. Deasphalted oil is solubilized in the solvent, and the insoluble pitch precipitates. Separation of a deasphalted oil phase, including the mixture of solvent and deasphalted oil, and a pitch phase, occurs in an extractor. The extractor separates the two phases and minimizes contaminants trapped in the deasphalted oil. The deasphalted oil phase is then heated to conditions where the solvent becomes supercritical to facilitate separation of the solvent and deasphalted oil, whereby the solvent can be recovered for recycling. Solvent deasphalting processes are described, for instance, in U.S. Pat. Nos. 4,816,140, 4,810,367, 4,747,936, 4,572,781, 4,502,944, 4,411,790, 4,239,616, 4,305,814, 4,290,880, 4,482,453, 4,663,028, and 7,566,394, all of which are incorporated herein by reference.
In addition, crude oil commonly contains organosulfur compounds and heteroatom compounds such as those containing nitrogen. These compounds are generally undesirable and must be removed during refinery operations. Light crude oil or condensates contain sulfur as low as 0.01 weight %. In contrast, heavy crude oil can contain up to about 3 weight % organosulfur. Similarly, the nitrogen content of crude oils is in the range of 0.001-1.0 weight %. The heteroatom and carbon residue (measured as Ramsbottom carbon residue, or RCR) content of various Saudi Arabian crude oils are given in Table 1, where “ASL” refers to Arab Super Light, “AEL” refers to Arab Extra Light, “AL” refers to Arab Light, “AM” refers to Arab Medium and “AH” refers to Arab Heavy.
TABLE 1PropertyASLAELALAMAHGravity, °51.439.533.031.127.6Sulfur, W %0.051.071.832.422.94Nitrogen, ppmw70446106414171651RCR, W %0.511.723.875.277.62Ni + V, ppmw<0.12.92134.067
The heteroatom content of crude oil generally increases with decreasing API gravity, or increasing heaviness, as is apparent from Table 1. The heteroatom content of the crude oil fractions also increases based on the boiling points of the fractions, as shown in Table 2.
TABLE 2Fractions, ° C.Sulfur WT %Nitrogen ppmwC5-900.01 93-1600.03160-2040.06204-2600.34260-3151.11315-3702.00253370-4302.06412430-4822.65848482-5703.091337
The discharge into the atmosphere of sulfur compounds during processing and end-use of the petroleum products derived from high organosulfur crude oil poses safety and environmental problems. In the industrialized countries of the United States, Japan and many countries of Europe, transportation fuel producers have already made necessary investments to reduce the sulfur content, and are producing environmentally clean transportation fuels. For instance, in 2007 the United States Environmental Protection Agency required sulfur content of highway diesel fuel to be reduced 97%, from 500 parts per million by weight (ppmw), referred to as low sulfur diesel, to 15 ppmw, referred to as ultra-low sulfur diesel. The European Union has enacted even more stringent standards, requiring diesel and gasoline fuels sold in 2009 to contain less than 10 ppmw of sulfur. Other countries are following in the footsteps of the United States and the European Union by establishing similar regulations that will require more refineries to produce low sulfur transportation fuels. The stringent sulfur specifications applicable to transportation and other fuel products have impacted the refining industry, and refiners must upgrade or replace existing refinery equipment to greatly reduce the sulfur content in fuel products.
Refractory sulfur compounds, which are considered very difficult to remove in hydrotreating processes conventionally employed for desulfurizing crude oil, include condensed-ring sulfur-bearing heterocyclic dibenzothiophene, shown below:
In addition, certain substituted dibenzothiophenes are particularly difficult to remove. A refractory sulfur compound, which is considered the most difficult to remove in processes employed for desulfurizing crude oil, include condensed-ring sulfur-bearing heterocyclic 4,6-dimethyldibenzothiophene, shown below:
4,6-dimethyldibenzothiophene can account for a significant percentage of the total organic sulfur in hydrocarbon mixtures such as whole crude oil. This compound can account for as much as 90 ppmw of the total sulfur content of Arabian Light crude oil, as much as 110 ppmw of the total sulfur content of Arabian Medium crude oil, and as much as 108 ppmw of the total sulfur content of Arabian Heavy crude oil. Although these concentrations are relatively low, 4,6-dimethyldibenzothiophene is very difficult to remove during the hydrotreating process at mild hydrotreating conditions, e.g., 30 Kg/cm2 pressure.
Oxidative desulfurization using liquid oxidizing agents in the presence of a catalyst, or combination of catalysts, is known to desulfurize dibenzothiophene and various substituted dibenzothiophenes including 4,6-dimethyldibenzothiophene, as well as other organosulfur compounds including, but not limited to, mercaptans and thiophenes. In these processes, the organosulfur compounds and, in certain processes, the organonitrogen compounds, are oxidized. The oxidation products are subsequently removed from the hydrocarbon product by extraction or other means. Oxidative desulfurization is described, for instance, in U.S. Pat. Nos. 6,160,193, 6,171,478, 6,274,785, 6,277,271, and 6,406,616, all of which are incorporated by reference herein.
It is desirable to remove at least some portion of the contaminants, including sulfur and nitrogen, during early processing steps in the refinery. Also, as discussed above, it is often necessary to desalt and deasphalt the crude oil during refinery operations in order to fraction the crude oil into useful products. However, conventional pretreatment desulfurization, desalting and deasphalting of crude oil generally requires separate and distinct process steps and associated unit operations equipment. For instance, in most refineries, desalting occurs upstream of atmospheric distillation, desulfurization is accomplished after distillation, and heavy distillation products are deasphalted. This means that during fractionating, a desalted stream, which contains organosulfur compounds and asphaltenes, is carried and subsequently converted or separated. Consequently, the distillation equipment and pre-distillation equipment must be sized and designed to accommodate the total volume, as well as chemical and physical attributes of a hydrocarbon mixture having asphaltenes and organosulfur compounds.
A combined process for the desalting and reducing the concentration of sulfur-bearing molecules is disclosed in U.S. Pat. No. 5,356,813. The crude, bacterial culture, oxygen and water are introduced into a stirred incubator vessel for a period of time and under conditions that permit the bacteria to cleave the sulfur-carbon bonds and form an inorganic, water soluble sulfate. In the process postulated, the bacteria is to be prepared by fermentation in a separate bioreactor under aerobic conditions that requires a nutrient medium which contains carbon, such as glycerol or glucose. As will be understood by one of ordinary skill in the art, the practice of the process disclosed in U.S. Pat. No. 5,356,813 requires biofermenting apparatus and associated technology not typically found in crude oil production and refining facilities. The desalting process is commonly performed in the field close to the wellhead or well collection point. Biofermenting apparatus are typically costly to acquire and maintain and require specific controls. In addition, engineering and technical personnel familiar with oilfield and refinery unit operations will not be knowledgeable about biochemical operations and equipment. It is also not clear whether the sole bacterial culture identified would be effective in the presence of the numerous other mineral salts that are typically found in crude oil. Furthermore, the temperature range recited in the patent of 10-60° C. for operation of the crude oil desalting and desulfurization vessel is substantially lower than the temperature of crude oil at the well head, and the cost of installing, operating and maintaining cooling equipment constitutes another disadvantage of the process. The requirement to reheat the crude oil for transport and subsequent processing is yet another cost factor detracting from practical commercial application of the process of U.S. Pat. No. 5,356,813.
Accordingly, a need exists for an efficient and effective method for desulfurization, desalinization and deasphalting of hydrocarbons, such as crude oil. As petroleum companies look to economize in light of increased processing costs, as well as more stringent worldwide regulations, this need becomes more urgent. The elimination or minimization of equipment presently used for desulfurization and desalinization, or consolidation of the existing equipment, to increase efficiency and lower costs, would be desirable.
Therefore, it is an object of the present invention to provide an integrated desulfurization, desalting and deasphalting process that can be practiced without addition to existing facilities of costly equipment, hardware and control systems.
It is another object of the present invention to provide an integrated desulfurization, desalting and deasphalting process that requires minimal modification of existing facilities.
It is still another object of the present invention to provide a hydrocarbon mixture to pre-distillation and distillation processes that has a reduced volume and a lesser chemical and physicochemical impact on existing processes.